A microscope faster than light
19 November 2020
Physicist Daniele Brida develops ultrafast lasers to follow in slow-motion chemical reactions and the inner working of electronic devices. This new kind of microscope allows the observation of phenomena at the nanoscale that were until now just too fast to be seen – improving photovoltaics and electronics devices.
It started with a simple question: “How do we see?”. We know it is thanks to our retina. It is covered with photoreceptor cells called cones, which are sensitive to colours, and rods, sensitive to low light. But how do rod cells function so sensitively and react when a single particle of light, a photon, reaches them?
The mechanism behind this feat is like a loaded spring ready to snap, explains Daniele Brida, a physicist at the University of Luxembourg:
“Rods are lined with a protein called visual purple, or rhodopsin. They are shaped like a cylinder. Inside it, a chain of carbon atoms unbends as soon it is hit by a photon. This starts a cascade of chemical reactions that ends up sending an electrical signal to the optic nerves.”
Faster is better
Around ten years ago, the physicist found experimental evidence of this theory by following the discharging of the loaded molecular spring. This experiment was possible thanks to an ultrafast laser built by Brida.
“One usually imagines that new instruments should be able to see smaller – or larger – things,” he says. “But looking at faster phenomena brings a crucial new understanding of chemical reactions or of the working of microchips.”
The Italian-born physicist keeps developing new systems. They resemble a forest of mirrors and lenses which split and redirect laser beams, all precisely arranged to generate extremely short pulses of light. The goal is to manipulate and visualize the motion of electrons, which constitute the basis of both chemistry and information processing devices.
His team at the University of Luxembourg can now shape laser pulses lasting around one femtosecond or one millionth of one billionth of a second (as a comparison, there are as many femtoseconds in one second as there are seconds in 30 million years).
These tools can help other scientists gain insight in fundamental processes involving the absorption of light, such as vision in animals, photosynthesis in plants and the conversion of solar energy into electricity in photovoltaics devices.
This is particularly important for organic solar cells, which are based on polymers as opposed to silicon as in standard solar modules. This technology could bring lower production costs and bendable modules, but is still struggling with low efficiency. Improving it will require studying exactly how the organic material generates free electrons when they absorb light.
Decades of work
Of course, a movie made with light cannot possibly be faster than light itself. Scientists use a trick: they carefully repeat the same experiment, again and again, recording a single measurement at a slightly different time. Once put to together, this data creates a time-lapse of the phenomena, a slow-motion movie somewhat reminiscent of a famous scene in the film “The Matrix” where the camera seems to circle around a flying bullet.
“We are building everything ourselves,” says the physicist. “At the beginning I used to do everything alone, as I thought I could do it quicker myself. But then I learned patience, to guide students and empower them. It is crucial because our work goes over decades and we need more and more people working on it. We are still at the start: we need to prove our approach really works. But then I believe we’ll open a whole new field.”
The electric light
Ultrafast microscopy can also be used to observe how electrons move across nanoscale devices. This could help with designing new microchips for high-speed electronics as well as developing quantum computers, a radically new type of information processing machine.
The devices developed at the University of Luxembourg exploit the fact that light is an electromagnetic wave, just like a radio signal or an X-ray. Because of that, it can be used as a source of a rapidly oscillating electric fields.
“We are now able to generate light pulses which are shorter than one oscillation of the electromagnetic wave. In this case, the system feels an electric field that grows, peaks, and decreases extremely rapidly. This allow us to use a laser as an electrode to study nanoelectronic devices. A first ultrashort laser pulse pushes electrons in a precise way, and a second pulse records how they move across the device.”
The physicist adds a deeper consideration:
“In our experiment, we sort of beat the Heisenberg Uncertainty Principle, a fundamental limit to what can be observed in a microscopic system obeying the laws of quantum mechanics. It is linked to the fact that looking at something will always change it, making it impossible to know it perfectly.”
We nevertheless found ways to really understand what is happening in the molecules we observe. Here, our work addresses deep questions of fundamental physics. They are not directly relevant for applications. But that’s also what motivates me to go to the lab.”
About the European Research Council (ERC)
The European Research Council, set up by the EU in 2007, is the premiere European funding organisation for excellent frontier research. Every year, it selects and funds the very best, creative researchers of any nationality and age, to run projects based in Europe. The ERC offers four core grant schemes: Starting, Consolidator, Advanced and Synergy Grants. With its additional Proof of Concept grant scheme, the ERC helps grantees to bridge the gap between grantees’ pioneering research and early phases of its commercialisation. https://erc.europa.eu/
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